ALPR systems generally include various image processing and computer vision techniques for: (1) locating a license plate within a scene, (2) segmenting characters on the plate, and (3) performing optical character recognition to determine the character sequence. In addition, determining the jurisdiction (e.g., such as the state associated with the license plate and the plate type) is often required. ALPR systems are used in electronic tolling, photo enforcement, parking, and other transportation applications where the vehicle identity is required.
Typical ALPR applications fix the choice of camera, lens, camera mounting location, vehicle trigger timing and distance, etc., at the design time to optimize the images sent for ALPR usage. Unfortunately, there are a number of noise factors, which cannot be fixed, that can substantially affect the performance of an ALPR system. These would include: the vehicle class (e.g., passenger car, large truck, motorcycle, etc.), the plate mounting location, the license plate design/layout, and the license plate size. For typical ALPR systems, these sources of variation often lead to images, which cannot be processed automatically and must instead be transmitted for human review.